Phantom Appearance of Non-phantom Matter in Bianchi Type-I Cosmological Model

Two cosmological models with non-phantom matter having the same expansion of the universe as phantom cosmologies are constructed in Bianchi type-I universe. The exact solutions to the corresponding Einstein field equations have been obtained. The cosmological parameters have been obtained in two interesting cases (i) γ=0 and (ii) γ=1/3. We have also discussed the well-known astrophysical phenomena, namely the look-back time, luminosity distance and event horizon with redshift.     

Phantom Appearance of Non-phantom Matter in Anisotropic Cosmological Model

Two cosmological models with non-phantom matter having the same expansion of the universe as phantom cosmologies are constructed in anisotropic Bianchi type-V universe. The exact solutions to the corresponding Einstein field equations have been obtained. The statefinder diagnostic pair i.e. {r,s} parameters have been obtained for disordered radiation i.e. γ=1/3. We have also discussed the well-known astrophysical phenomena, namely the look-back time, luminosity distance and event horizon with redshift.     

Quantum fluctuations near the classical space-time singularity

The method of path integration is used to study the effects of quantum fluctuations in the space-time geometry near the classical singularity of general relativity. It is shown that in certain special cases explicit Feynman propagators can be constructed which enable us to evaluate these fluctuationsquantitatively. The cases discussed are (i) the gravitational collapse of a uniform dust ball, (ii) the Friedmann cosmologies, (iii) the axisymmetric Bianchi type I cosmological model, and (iv) the general anisotropic Bianchi type I cosmological model. In all cases discussed here the quantum uncertainty grows to infinity as the classical space-time singularity is approached. In this wider regime of quantum gravitation nonsingular solutions can occur with finite probabilities.     

Assisted Inflation in Bianchi VI0 Cosmologies

Exact models for Bianchi VI₀ spacetimes with multiple scalar fields with exponential potentials have been derived and analysed. It has been shown that these solutions, when they exist, attract neighbouring solutions in the two cases corresponding to interacting and non-interacting fields. Unlike the results obtained in a previous work dealing with the late-time inflationary behaviour of Bianchi VI₀ cosmologies, the knowledge of exact solutions has made possible to study in detail the occurrence of inflation before the asymptotic regime. As happened in preceding works, here as well inflation is more likely to happen with a higher number of non-interacting fields or a lower number of interacting scalar fields.     

Asymptotic Regimes of Magnetic Bianchi Cosmologies

We consider the asymptotic dynamics of the Einstein-Maxwell field equations for the class of non-tilted Bianchi cosmologies with a barotropic perfect fluid and a pure homogeneous source-free magnetic field, with emphasis on models of Bianchi type VII₀, which have not been previously studied. Using the orthonormal frame formalism and Hubble-normalized variables, we show that, as is the case for the previously studied class A magnetic Bianchi models, the magnetic Bianchi VII₀ cosmologies also exhibit an oscillatory approach to the initial singularity. However, in contrast to the other magnetic Bianchi models, we rigorously establish that typical magnetic Bianchi VII₀ cosmologies exhibit the phenomena of asymptotic self-similarity breaking and Weyl curvature dominance in the late-time regime.     

量子纠缠与宇宙学弗里德曼方程

量子纠缠作为量子信息理论中最核心的部分,代表量子态一种内在的特性,是微观物质的一种根本的性质,它是以非定域的形式存在于多子量子系统中的一种神奇的物理现象.熵也是量子信息理论的重要概念之一,纠缠熵作为量子信息的一个测度已经成为一种重要的理论工具,为物理学中的各类课题提供了新的研究方法.本文主要考虑量子纠缠的宇宙学应用,试图更好地从纠缠的角度来理解宇宙动力学.本文研究了量子信息理论的概念和宇宙学之间的深层联系,利用费米正则坐标和共形费米坐标构建了弗里德曼- 勒梅特-罗伯逊-沃尔克宇宙学弗里德曼方程和纠缠之间的联系.假设小测地球（a geodesic ball）的纠缠熵在给定体积下是最大的,可以从量子纠缠第一定律推导出弗里德曼方程.研究表明引力与量子纠缠之间存在着某种深刻的联系,这种联系对引力场方程的解是成立的.     

Inhomogeneous generalization of some Bianchi models

Vacuum Bianchi models which can be transformed to the Einstein-Rosen metric are considered. The models are used in order to construct new inhomogeneous universes, which are generalizations of Bianchi cosmologies of types III, V and VI_h. Recent generalizations of these Bianchi models, considered by Wainwright et al., are also discussed.     

Singularities in Bianchi cosmologies

We discuss how infinite density singularities may be shown to occur in Friedmann-Robertson-Walker universes and orthogonal spatially homogeneous universes, but how very different behaviours are possible in tilted homogeneous cosmologies. After considering various possibilities that arise in this case, we illustrate them by examining the behaviour of exact solutions of Einstein's equations for a homogeneous cosmology which is a locally rotationally symmetric tilted Bianchi type V universe. These universes - which can be arbitrarily similar to a Robertson-Walker universe at late times - show a variety of singular behaviours quite different from those in the ‘orthogonal’ case. In particular, there exist such universes in which two singularities occur at the early stages of the universe, but in which the density of matter is finite at all times.     

Conformal Killing Vectors in LRS Bianchi Type Ⅴ Spacetimes

In this note,we investigate conformal Killing vectors(CKVs)of locally rotationally symmetric(LRS)Bianchi type V spacetimes.Subject to some integrability conditions,CKVs up to implicit functions of(t,x)are obtained.Solving these integrability conditions in some particular cases,the CKVs are completely determined,obtaining a classification of LRS Bianchi type V spacetimes.The inheriting conformal Killing vectors of LRS Bianchi type V spacetimes are also discussed.     

Conformal Killing Vectors in LRS Bianchi Type V Spacetimes

In this note, we investigate conformal Killing vectors (CKVs) of locally rotationally symmetric (LRS) Bianchi type V spacetimes. Subject to some integrability conditions, CKVs up to implicit functions of (t,x) are obtained. Solving these integrability conditions in some particular cases, the CKVs are completely determined, obtaining a classification of LRS Bianchi type V spacetimes. The inheriting conformal Killing vectors of LRS Bianchi type V spacetimes are also discussed.     

Is the cosmological singularity thermodynamically possible?

The four broad approaches that have been suggested heretofore to eliminate the initial singularity from cosmology are briefly reviewed. None is satisfactory, basically because one does not know enough about the microphysics involved in the process. Thermodynamics has often been used in such dilemmas, and it is proposed to answer the question of whether there was a Friedmann-like singularity in the universe by exploiting the bound on specific entropy that has been established for finite system. It is made applicable to the universe by considering only a causally connected spacelike region within the particle horizon of a given observer. It is found that the specific entropy of radiation in such a region can exceed the bound if the observer is too early in the universe. Faith in the bound leads to the conclusion that the Friedmann models cannot be extrapolated back to nearer than a few Planck-Wheeler times from the singularity. The Friedmann initial singularity thus appears to be thermodynamically unacceptable.     

Cosmic microwave background spectrum and G-varying cosmology

It is shown that G-varying cosmologies provide a better fit to the observed data on cosmic microwave background, than the standard Friedmann models.     

Anisotropic Bianchi Type-II Massive String Cosmological Models with Time-Decaying Λ Term and Thermodynamic Parameters

The present paper envisages a spatially homogeneous and anisotropic Bianchi II massive string cosmological models with time-decaying Λ term in general relativity. By using the variation law of Hubble’s parameter, the Einstein’s field equations have been solved for two general cases. The first case involving a power law solution describes the dynamics of universe from big bang to present epoch while the second case admit an exponential solution seems reasonable to project dynamics of future universe. We observed that massive strings dominate in early universe and eventually disappear at late time, which is consistent with the current astronomical observations. It has been found that the cosmological constant (Λ) is a decreasing function of time and it approaches to small positive value at sufficiently large time. The thermodynamic properties of anisotropic Bianchi II universe are studied and also the absolute temperature and entropy distribution are given explicitly. The relations between thermodynamic parameters and cosmological constant Λ has been established. Physical behavior of the derived model is elaborated in detail.     

Finite temperature quantum fields in expanding universes

The thermodynamics of an ideal relativistic quantum gas in expansion is studied. It is found that only for conformally invariant fields in conformally static spacetime can thermal equilibrium be strictly maintained. A finite temperature theory can be defined under the condition of quasi equilibrium when the background expansion is nearly adiabatic. The high temperature expansion of the energy density for massive nonconformal fields in Robertson-Walker universes and for conformal fields in Bianchi Type-I universes are calculated. The importance of these results on phase transition and quantum processes in the early universe is discussed.     

Exact Bianchi type V imperfect fluid solutions

It is shown that it is sufficient to solve a single first order differential equation to determine exact Bianchi type V imperfect fluid solutions of Einstein's field equations. Two exact solutions are presented in this paper. One of these two solutions corresponds to the case when the anisotropic pressure tensor is proportional to the shear tensor. This proportionality, it is shown, is a necessary but not a sufficient condition for the absence of heat flow in a Bianchi type V universe. It is also observed that the presence of heat flow necessarily introduces anisotropy in a Bianchi type V universe.     

离散时空的Friedmann宇宙的几何结构及其性质

研究了离散时空的Friedmann宇宙的几何结构,证明了形成Friedmann宇宙的尘埃物质并不分布在Friedmann时空点上,导出了在Friedmann宇宙中的试验粒子的测地运动方程,并揭示了Friedmann宇宙中的时钟和频移效应. 关键词： Friedmann时空 尘埃物质 测地运动 时频效应     

Holographic Dark Energy Model with Quintessence in Bianchi Type-I Space-Time

In this paper, we have studied a homogeneous and anisotropic universe filled with matter and holographic dark energy components. Assuming deceleration parameter to be a constant, an exact solution to Einstein’s field equations in axially symmetric Bianchi type-I line element is obtained. A correspondence between the holographic dark energy models with the quintessence dark energy models is also established. Quintessence potential and the dynamics of the quintessence scalar field are reconstructed, which describe accelerated expansion of the universe.     

The effect of spin generated torsion on the early universe's evolution

The earliest phase of cosmological evolution is studied in a universe where spin-generated torsion effects are important. The evolution of the scale factor is studied in cosmologies with a Robertson-Walker line element and a spin density aligned in the radial direction (±r). Solutions are found for the cases of stiff matter, radiation, pressureless matter and decompressive matter admitting a spin density.